Either this event or the Triage event will spawn on this particular map tile. At other times, no event will spawn at all. The event is initiated by simply passing by, and it's usually a surprise that springs itself upon the player.

The small area contains guards from Bastion's Keep attempting to ward off a large number of Khazra invaders, including the enraging mutated goatmen, and the spearmen who love to lurk around the periphery taking potshots at the player and fleeing constantly. The first phase of the event involves simply killing these beasts.
The monsters that are part of the event will have a golden circle around their feet, and will show up as red dots on the minimap (similarly to how elite monsters do). The melee Blood Clan Warriors tend to enrage at the same time, which increases their size and attack speed while also granting them a powerful knockback attack that can chain and basically stun a player for large periods of time. The knockback also causes the player to bring additional monsters into the fray. The spearmen and moloks, due to their fleeing, also inevitably force the player to add large groups of monsters from the Battlefields into the event.

Heralds of Pestilence often are around the area as well, and can very quickly kill a player even when they aren't on the screen. With the knockback attack from the warriors, this becomes an incredibly deadly situation that will need to be dealt with immediately. Focusing on the heralds, as with any other area they inhabit, is key to victory.

Once the khazra have been defeated, a very large molok superunique will spawn, along with a group of normal moloks (who are not elite minions). The colossal firewing has the molten trait, leaving behind a harmful trail of fire, and will explode moments after death. In addition, the firewing has a breath of fire attack similar to that seen on Oppressor enemies.

Dispose of them, and then speak to Colonel Severyn to be done with the event, and savor the not-worth-it rewards.